1. Field of the Invention
The present invention relates to a system for automatically controlling the concentration of developer solution in a photocopying machine. The developer solution typically comprises toner concentrate, which includes chargeable toner particles, and a vehicle or carrier with which the concentrate is combined.
Photocopying machines are now in widespread use for reproducing all forms of documents. These machines are usually either of the "single component" dry toner type or "double component" wet toner type. In the machines of the second type, a copy sheet having a photoconductive coating is uniformly electrostatically charged. The image of a document to be copied is then projected onto the charged copy sheet while at an imaging station. A conductive ground plate, forming part of the imaging station, is effective to allow discharge of selected areas of the sheet in accordance with the projected image. In this way, a latent electrostatic image is formed on the sheet. The latent image is developed by passing the sheet through a trough containing a toner or developer solution that carries a large number of particles charged oppositely to the charge of the latent image. Accordingly, the particles are attracted to the image areas on the sheet to later be fixed and dried in a well known manner and thereby yield a finished copy.
Toner particles are, of course, removed from the developer solution each time a copy is made. Therefore, if no toner concentrate is added to the solution, the concentration of toner particles therein decreases. However, if acceptable copies are to be made, that is, copies which are adequately developed, the toner concentration should be maintained above a predetermined minimum level. But further, if the toner concentration exceeds certain levels, the copies made will be smudged or grainy. Therefore, the solution concentration must also be maintained below certain maximum levels.
The system of the present invention automatically and accurately controls the toner particle concentration of a developer solution in an electrostatic photocopying machine.
2. Description of the Prior Art
The problem of maintaining the toner particle concentration of the developer solution in an electrostatic photocopying machine within an acceptable range has been considered in the past. For example, some systems intended to provide such control continually add toner particles or toner concentration to the developer while the machine is in operation at a rate correlated to an average rate of particle removal. However, if the copies being made withdraw particles at a rate which deviates substantially from the average rate, then the developer solution cannot be maintained at its preferred concentration.
One automatic system adds toner concentrate to the developer solution until sufficient solution concentration is detected. At that time the concentrate adding mechanism is disabled. However, if the detection device fails, the solution becomes too concentrated or overflows its holding tank. Thus, the systems described above have certain drawbacks.
Another approach to photocopying machine developer solution concentration control is disclosed in U.S. Pat. No. 3,739,800 (Aasen et al.) which describes a system that includes a photocell for detecting the need for additional toner concentrate. The photocell receives varying amounts of light transmitted through a transparent monitoring tube through which the developer solution circulates. If the solution is not concentrated enough, the resistance of the photocell lowers to a level that permits a periodically generated enabling pulse from an electronic control circuit to actuate a solenoid valve. Short timed bursts of toner concentrate are added to the developer solution by actuation of the solenoid at preselected intervals until the concentration of the solution reaches an acceptable level.
The Aasen et al. device also has certain drawbacks. In particular, the amount of toner concentrate added at each interval during which the solenoid is actuated is not controlled nor is the frequency of the intervals altered. The only variable controlled by the device is the number of short bursts of concentrate which are added. Therefore, control of the developer solution concentration is not precise.